Japanese Laid-open Patent Publication No. 2007-19748, for example, discloses a system in which a mobile telephone unit sounds an alarm when the mobile telephone unit and a slave unit are equal to or more than a certain distance away from each other by making the mobile telephone unit having a receiving module and the slave unit communicate with each other periodically, as a measure for preventing misplacement of the mobile telephone unit, which is one example of the radio communication device. The mobile telephone unit disclosed in Japanese Laid-open Patent Publication No. 2007-19748 serves as a base unit in relation to the slave unit.
The system is configured as follows. The slave unit transmits a radio wave, and the base unit receives the radio wave from the slave unit and observes the signal strength of the received radio wave. When the signal strength of the received radio wave becomes less than a certain level, an alarm is sounded. The signal strength of the received radio wave is referred to as “RSSI” (Received Signal Strength Indication).
In actual use of the conventional system as described above, it is expected that the RSSI considerably decreases instantaneously in conditions where, for example, the base unit is placed on metal or where the radiation patterns of the base unit and the slave unit antennas have a null relationship to each other. The reason is that the antenna characteristics of the base unit considerably deteriorate by placing the base unit on metal. In the conventional system as described above, the RSSI may even become less than a certain level, that is, less than a threshold value, in other circumstances than when the base unit is misplaced. Thus, the conventional system has the problem where a false alarm is caused by a malfunction. The conventional system described in Japanese Laid-open Patent Publication No. 2007-19748 cannot cope with an abrupt variation of the RSSI, so the conventional system cannot resolve the problem of the false alarm caused by the malfunction.